As the world-wide demand for energy increases exponentially, there is a heavy burden placed on traditional sources of energy, such as non-renewable fossil-based fuels. The price of crude oil has shown significant historical fluctuations. The spiraling cost of crude oil in recent years adversely effects the bottom-line of many small and medium scale energy-intensive industries such as foundries and the like. Therefore, alternate sources of energy, e.g., solar power, have become increasingly attractive in recent times. There is an environmental benefit to adopting cleaner energy sources as reduction of burning of fossil fuels helps to reduce greenhouse gas emissions
Solar energy technology has developed significantly since the ancient Greek civilization learned the art of concentrating sun's rays. Over the years, solar energy research has helped develop systems that have improved efficiency and are more economical. However, a dearth of information, materials, complexity, and manufacturing skills remain an impediment to large-scale production and utilization of this abundantly available energy source.
Solar concentrators are characterized and classified based on several criteria including the means of concentration: reflection or refraction; type of focusing: point, line, or non-focusing; type of concentrator: fixed or tracking concentrator; and, type of receiver: fixed or tracking receiver.
As will be described and illustrated hereinafter, there are three primary types of reflective solar concentrating systems based on the type of focusing they produce, viz., (a) parabolic trough concentrators that produce line focus; (b) central receiver-type concentrators that concentrate sunlight onto distant and centralized towers; and, (c) parabolic dish concentrators that produce a point focus.
Based on the end application, the different types of solar concentrators are employed to achieve optimum results. For example, the parabolic trough concentrators are employed to produce electricity and heat. However, troughs have a low maximum solar concentration, high receiver heat loss, and are expensive. Central receiver-type concentrators are typically employed in large scale applications for electricity generation and require vast real-estate for proper deployment and are thus not economical for small and medium-scale industries. On the other hand, parabolic dish concentrators are highly efficient in concentrating the sun's rays. The biggest impediment to parabolic dish concentrators include, inter alia, the prohibitive costs associated with compound and complex reflector curves and expensive mirror substrates.
In a typical solar concentrating system used in furnace applications, a high energy density solar radiation is provided to a target receiver, thus raising the temperature of the target. Depending on the degree of concentration, the optical properties (solar absorption and radiation) of the target surface, the system may be utilized to melt a target surface, thus forming a solar furnace.
Over the years attempts have been made to design and construct solar concentrators that provide point focus (high solar concentration) with minimum complexity and cost. U.S. Pat. No. 5,374,317 (Lamb et al.) discloses a multiple reflector concentrator solar electric power system. In this system, the sun's rays first reach a plane of individual primary reflectors (which may be flat or curved). The primary reflectors then reflect the solar radiation to the location of secondary reflectors (which may again be flat or curved) and are then passed through to the photovoltaic component. The system disclosed by Lamb et al. uses a large number of components other than primary and secondary reflectors such as tertiary reflectors, optional cover plates, and heat dissipation components particularly suited for solar power generation. This results in a system that is complex and expensive.
U.S. Pat. No. 6,530,369 (Yogev et al.) also describes a system comprising two reflectors that are successively arranged along an optical path of the system so that the first of the two reflectors reflects the radiation towards the second reflector. The concentrated radiation from the second reflector is directed to a solar receiver. However, the second reflector is realized as a tower reflector. As discussed earlier, central receiver-type concentrators are typically employed in large scale applications for electricity generation and require vast real-estate for proper deployment and are thus not economical for small and medium-scale industries.
International Patent Publication No. WO 2005/022047 A2 (Shifman) discloses a solar energy utilization unit comprising a solar radiation concentrating component and a solar energy receiving component. The concentrating component comprises a concave primary reflector and a convex secondary reflector, for concentrating incident solar radiation and forwarding the concentrated radiation into the receiving component. However, the reflectors are dish-shaped and require high precision curved surfaces for obtaining proper concentration effects.
U.S. Pat. No. 4,784,700 (Stern et al.) describes a point focus solar concentrator which uses various geometries of cylindrical reflector strips as primary and secondary reflectors to simulate a point focus by overlapping the line foci of each segment at a coincident point. Although, the Stern et al. device uses cylindrical rather than dish-shaped parabolic mirrors that are easier to form and polish since they have a simple curvature, the arrangement of the reflector strips in a Fresnel-type mirror arrangement requires high manufacturing skills and therefore increases its cost.
U.S. Pat. No. 3,118,437 (Hunt) discloses a system of two reflective surfaces or two sets of reflective surfaces facing each other in an arrangement that causes all rays striking the first reflective surface to converge onto a substantially one point or limited area. Although Hunt discusses a system of two reflective surfaces, wherein the effective axes of curvature of a second surface or set of surfaces are being normal to the effective axes of curvature of the first surface or set of surfaces, the practical embodiments of Hunt's reflective system are complex and require elaborate infrastructure, such as, for example, carriages and tracks.
There is accordingly a need for an improved solar concentrating system that overcomes the limitations associated with using complex construction requiring high degree of skills. Moreover, there is a need for an improved solar concentrating system wherein the prohibitive costs associated with manufacture and deployment of a traditional solar concentrating system are minimized thereby making it attractive for use by small and medium scale energy-intensive industries.